The tilt, the seasons, and why 0° Aries is a moment

Derive the four cardinal points of the zodiac from the Earth's axial tilt, and explain what that same tilt sets a hard ceiling on.

Lesson 2 of 10 in Astronomy for Astrologers · free to read · no account, no email

Twenty-three and a half degrees

The Earth's axis is not upright with respect to its orbit. It leans by about 23°26′, and — this is the part that matters — it keeps pointing the same way all year, like a spinning top carried around a room. In June the northern end leans toward the Sun; in December it leans away. Nothing about the axis changed. The Earth moved.

That lean is the whole cause of the seasons, and it is worth saying plainly because the wrong answer is so common: seasons are not about distance from the Sun. The Earth is actually closest to the Sun in early January, in the middle of the northern winter. What changes is the angle the light arrives at and how long it stays up.

Now project the Earth's equator outward onto the sky and you get the celestial equator. Project the Earth's orbit outward and you get the ecliptic. Two great circles, tilted from each other by that same 23°26′, which astronomers call the obliquity.

Two circles crossing at an angle meet in exactly two places, and they run furthest apart exactly halfway between. Four special points. You already know their names.

Four moments, four numbers

At the two crossings, the Sun is exactly on the celestial equator: day and night are equal everywhere. Those are the equinoxes. The one where the Sun is heading north is defined as 0° Aries. The one six months later, heading south, is 0° Libra.

At the two extremes, the Sun is as far from the equator as it can get — 23°26′ north, then 23°26′ south. Those are the solstices, and they are 0° Cancer and 0° Capricorn.

So the four cardinal points of the zodiac are not places among the stars. They are events, defined by the geometry of a tilted planet going round a star. That is the single most useful sentence in this module. It is why Western astrology is called tropical — from the Greek for “turning”, the solstice being the point where the Sun turns back.

And it means the modalities were never asserted. They were derived. Cardinal signs begin seasons because the Sun entering Aries is the equinox; the sign does not start near the season's beginning, it starts exactly at it. Fixed signs hold the middle of a season, mutable signs cover the turning. Module 1 taught you that as a pattern. Here is where it comes from.

What the tilt puts a ceiling on

The obliquity is not only the source of the cardinal points. It is a limit, and the limit has a name you have probably already seen badged on a chart.

Because the Sun rides exactly on the ecliptic, its declination — how far north or south of the celestial equator it is — can never exceed the obliquity. The Sun's furthest reach is 23°26′ and not one arcminute more, ever. The other bodies are not on the ecliptic, so they can and sometimes do go further. When one does, it is out of bounds: past where the Sun itself can reach. The whole meaning of that badge is this one number.

The tilt has a second consequence you will meet in the next module. Because the ecliptic is tilted with respect to the horizon, the twelve signs do not take the same length of time to climb over it. At middling northern latitudes some signs take well over two hours to rise and others take well under one, which is why Rising signs are not evenly distributed across a population and why house systems start to argue with each other as you go north.

One last thing, for precision's sake: the obliquity is not a constant. It is slowly shrinking, by roughly 47 arcseconds a century — it was about 23°42′ two thousand years ago. A chart cast for an ancient date uses the obliquity of that date, and Umbriel's engine does. It is a small number that quietly separates careful software from the other kind.

What that looks like

Take any June solstice. The Sun's declination that day is +23°26′, its maximum, and it is at 0° Cancer by definition — the two facts are the same fact. Six months later it is −23°26′ at 0° Capricorn. Halfway between, at both equinoxes, its declination is 0° and it sits at 0° Aries or 0° Libra. Four numbers, one tilt. You could rebuild the skeleton of the tropical zodiac from a protractor and a calendar.

What to take away

• The seasons come from the Earth's 23°26′ axial tilt holding its direction all year, not from distance to the Sun.
• The celestial equator and the ecliptic cross at that angle, and the four points where they cross or run furthest apart are 0° Aries, Cancer, Libra and Capricorn.
• The cardinal points are moments, not places — which is what “tropical” means, and where the modalities actually come from.
• The Sun's declination can never exceed the obliquity, so “out of bounds” means simply: further from the equator than the Sun can go.

The common mistake

Explaining the seasons by distance from the Sun. It is the most common wrong answer there is, and it falls apart the moment you notice that the northern and southern hemispheres have opposite seasons at the same moment while sharing one distance.

Try it on your own chart

Look through your own chart for an out of bounds marking. If you have one, the number it beat is the number in this lesson: your planet went further north or south than the Sun has ever managed. If you do not have one, look at your Sun's degree instead and work out how many days you were born from the nearest equinox or solstice — the Sun covers roughly a degree a day, so the degree number is nearly the answer.

Check yourself

The same questions the lesson asks in the app. Open one to see the answer.

Keep going

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